1. Field of the Invention
The invention is directed to an improved fuel injection system for internal combustion engines.
2. Description of the Prior Art
One fuel injection system of the type with which this invention is concerned is known for instance from German Patent Disclosure DE 197 01 879 A1 and includes a fuel tank, from which fuel is pumped into a high-pressure collection chamber by a high-pressure pump. In the high-pressure collection chamber, a predetermined high fuel pressure is maintained by means of a regulating device. From the high-pressure collection chamber, high-pressure supply lines corresponding in number to the number of combustion chambers of the engine each lead to one fuel injection valve, and the fuel injection valve can be made to communicate with the high-pressure supply line by a control valve. For reasons of space, the control valve and the fuel injection valve are often disposed in one housing. The fuel injection valve here includes a valve needle, which is guided in a bore and is surrounded, in the region toward the combustion chamber, by a pressure space. A pressure face is embodied on the valve needle and is acted upon by the fuel in the pressure space, so that when a certain opening pressure in the pressure space is reached, the valve needle executes a longitudinal motion counter to a closing force and thus opens at least one injection opening, through which fuel from the pressure space reaches the combustion chamber of the engine. The control valve of the fuel injection system is embodied as a 3/2-way valve, which in one position makes the high-pressure collection chamber communicate with the pressure chamber of the fuel injection valve, and in a second position interrupts the communication with the high-pressure collection chamber and causes the pressure chamber to communicate with a leak fuel chamber embodied in the valve body, which leak fuel chamber communicates with the fuel tank via a line, so that a low fuel pressure always prevails in the leak fuel chamber. If the control valve switches from the closed position to the opened position, a pressure wave is created, which passes through the inlet conduit into the pressure space, where it causes a pressure advantage; that is, the injection of the fuel takes place at a pressure which is markedly higher than the pressure in the high-pressure collection chamber. As a result, high injection pressures are obtained at a moderate high pressure in the high-pressure collection chamber and in the parts of the fuel injection system that carry the high fuel pressure. Since the fuel in the supply lines is in motion through the opened control valve during the injection, it is stopped abruptly upon closure of the control valve, and so the kinetic energy of the fuel is converted into compression work. This creates pressure fluctuations, which upon a second injection immediately following the first makes precise and exact metering of the injection quantity difficult, since because of the pressure fluctuations, the state of the control valve is not precisely known.
It is therefore the object of the present invention to construct a fuel injection system such that precise metering of the injection quantity and precisely definable main injections, preinjections and postinjections are made possible.
The fuel injection system of the invention has the advantage over the prior art that the pressure fluctuations that occur upon closure of the control valve, that is, upon the interruption of the communication with the high-pressure collection chamber, are damped by the communication of the first pressure space or the high-pressure supply line with a damping chamber via a throttle, and thus fade quickly. After closure, the control valve therefore quickly regains a steady state, making it possible within a short time interval from the preceding injection to perform a second injection and thus to control its injection quantity quite precisely. The control valve is a 3/2-way valve in a control valve body and contains a control valve member, which is longitudinally displaceably guided along a control bore. By radially widening the control bore, two pressure spaces are embodied in the control bore; the first pressure space communicates with the high-pressure collection chamber, and the second pressure space communicates with the pressure chamber embodied in the fuel injection valve. In the closing position of the control valve member, the communication between the first and second pressure space is interrupted, and the second pressure space and thus the pressure chamber communicate with a leak fuel chamber and are thus pressureless. In the opening position of the control valve member, the communication between the first and second pressure space is opened, and the communication of the second pressure space with the leak fuel chamber is interrupted, so that the high-pressure collection chamber communicates with the pressure chamber.
The first pressure space communicates with a damping chamber via a throttle, and so pressure fluctuations of the kind that occur upon opening and closure of the control valve in the first pressure space and also in the high-pressure supply line are damped. By a suitable design of the throttle, the damping characteristic can be selected such that pressure fluctuations in the pressure space already fade completely after only a few fluctuation periods.
In a first advantageous feature of the subject of the invention, the damping chamber is embodied as a bore, which extends in the valve holding body parallel to the longitudinal axis thereof. As a result, the damping chamber can be realized in the already known fuel injection valves without rebuilding, and without having to change the outer diameter of the fuel injection valve.
In a further advantageous feature, the valve holding body is axially braced against the control valve body with the interposition of a shim. The bore forming the damping chamber extends partly inside the control valve body, through the shim, and for a greater part in the valve holding body. The throttle is embodied in the shim, so that by replacing the shim with one having a different throttle, the fuel injection valve can be adapted to given requirements without having to make structural changes in the rest of the fuel injection valve.
In a further advantageous feature of the subject of the invention, the damping chamber comprises two parallel bore portions, both extending in the valve holding body. The two bore portions of the damping chamber communicate with one another through a transverse conduit, so that a shorter valve holding body can be achieved, for the same volume of the throttle bore.
In a further advantageous feature, the two bore portions of the damping chamber communicate through a transverse conduit which is disposed in a shim that in turn is disposed between the valve holding body and the valve body. As a result, a transverse connection of the bore portions inside the valve holding body, which can be fabricated only at relatively great effort and expense, for instance using an end-milling cutter, is unnecessary. Embodying the transverse connection in the shim makes it possible for both bore portions of the damping chamber to be embodied originating on one of the face ends of the valve holding body.
In a further advantageous feature, at least two throttles are disposed in the line that connects the damping chamber with the high-pressure supply line. As a result of the two throttles, a markedly more powerful throttling is obtained than with only one throttle, so that the two throttles can have a substantially larger flow cross section than a single throttle with the same damping action. This makes the risk that the throttles will become plugged with dirt particles in the fuel much less. It is especially advantageous for the two throttles not to be disposed in a line, aligned with one another, but offset radially from one another, which additionally reinforces the damping action.
In a further advantageous feature of the subject of the invention, a closing valve is disposed between the damping chamber and the first pressure space; it opens the communication between the first pressure space and the damping chamber only whenever damping is desired. The pressure advantage upon opening of the control valve that is desired for the sake of injection at the highest possible pressure is reduced somewhat because of the constant communication between the first pressure space and the damping chamber. The closing valve therefore interrupts the communication between the first pressure space and the damping chamber during the opening phase of the control valve. After the termination of injection, the closing valve is opened, so that the pressure waves are damped quickly, as before, in the first pressure space. Thus by means of this closing valve, an optimal injection pressure and simultaneously a damping of the pressure fluctuations are obtained, which makes exact metering of the injections possible.
In another advantageous feature, the closing valve is controlled by the pressure in the second pressure space. With the control valve opened, at least approximately the same pressure prevails in the second pressure space as in the first pressure space, and the closing valve is closed by that pressure. If the control valve closes the communication between the first pressure space and the second pressure space, then the pressure in the second pressure space drops, and the closing valve as a result opens the communication between the first pressure space and the damping chamber. The damping of the pressure fluctuation then ensues as already described. The control by the pressure in the second pressure space makes an additional electronic triggering of the closing valve unnecessary.
In a further advantageous feature of the subject of the invention, the control valve body is fabricated from a hard steel, while the valve holding body in which the damping chamber is embodied is fabricated from a relatively soft steel. The control valve, which contains sealing faces that are subjected to severe stress, is disposed in the control valve body. Because it is made of a hard steel, wear in the region of the valve seat of the control valve is reduced. To embody the valve holding body, conversely, a soft steel is advantageous, since no seat or sealing faces are provided in it, and thus there is no severe mechanical stress. The hollow chamber that forms the damping chamber can be made economically and quickly in the soft steel.